The present invention relates generally to a battery venting system that is useful for cordless power tools. The system includes one or more of battery cells, sleeves that may surround the battery cell, a carrier that can position and hold a number of battery cells to form a cell pack, and a charger useful with the battery cells and/or carrier.
Rechargeable nickel-cadmium cells and nickel-metal hydride cells are widely used as a battery power source for portable apparatus, such as power tools. Typically, nickel-cadmium cells or nickel-metal hydride cells are used in the form of a cylindrical cell that has a cylindrical shape. In the portable apparatus, the cylindrical cells are used alone or more typically in the form of a cell pack in which a plurality of cells are connected and are contained within a housing that can be removably attached to an apparatus.
FIG. 1 shows a typical cylindrical cell, which has a cylindrical case 1 that contains the cell materials in a closed fashion by a cover 11. A conductive projection 11a is formed in the cover 11, and a safety valve for releasing gasses is disposed in the projection 11a. Generally, the cover 11 having the projection 11a the positive electrode and the case 12 is the negative electrode. A sleeve 13 usually surrounds the peripheral side of the cell and is formed from an electrically insulating material so that when the battery cells are intentionally or unintentionally touching each other at the peripheral side face, the battery cells will not short out or discharge.
When the cylindrical cell 1 is to be used as a power source for an apparatus, a plurality of cells are connected to each other to form a cell pack, as shown in FIG. 2. Adjacent cells 1 are bridged by an electrically conductive plate 9, such as a nickel plate. The plate may be attached by spot-welding or other methods of attaching to the projection 11a of the cover 11 of one of the cells and the bottom face of the case 12 of the other cell, respectively. To further secure and position the plurality of cells, it is known to wrap the periphery of the outermost cells with a shrinkable plastic or tape.
A disadvantage to providing an electrically insulating case around each cell is that when a plurality of cells are provided to form a cell pack, heat generated upon discharge and upon recharging operations is not easily dissipated. Likewise, when a cell pack is wrapped with shrinkable plastic or tape, the generated heat is further hindered from dissipation. When the temperature of the cell is raised as a result of the heat generation, self-discharge may reduce the cell capacity or shorten the service life of the cell. This problem is also applicable to sealed-type cells or other types such as rectangular cells.
Accordingly, there have been attempts to address this issue by providing cell holders such as that shown in U.S. Pat. No. 5,578,392, particularly FIG. 13. There, a cell holder is provided to hold and position individual battery cells. In this cell holder, an upper and a lower plate is provided to respectively receive the upper and lower portion of the individual cells. Each plate has a plurality of spaced apart cell holes into which a portion of the upper or lower portion of a battery cell can be received. Accordingly, a gap is formed between adjacent cells to facilitate heat dissipation during discharge and charging operations. Although this solution may be useful, the plates take up space within the battery pack housing and by virtue of their size still hinder heat dissipation.